None.
The present invention relates to a radar altimeter, and more particularly to a low probability of intercept (LPD coherent radar altimeter.
Existing radars with high performance LPI capability, high altitude operation, frequency agility, transmission power management, and a phase coded spread spectrum transmission format arc large multi-board assemblies. It would be desirable for a radar altimeter to digitize all control and signal processing functions, such that the system could be reduced to a single digital signal processor (DSP) chip and a couple of radio frequency (RF) hybrid circuits. For example, modulation of the transmission signal is normally done with discrete analog circuitry. By moving the modulation process into the DSP, numerous control circuits and interface leads are eliminated. By performing each function digitally in the DSP, the function is not only eliminated from the discrete printed circuit board (PCB) circuitry, but its associated control-interface circuit and interface pin-outs arc also eliminated. Further, by running the digitizer at a high sampling frequency (e.g., 120 MHz), normal high resolution enhancement functions are eliminated (e.g., programmable delay lines, RF wide-band gate and correlation functions).
A problem that occurs in existing radar altimeters is frequency drift of the transmission signal with respect to the center frequency of processing filters. Solving this problem requires either a relatively elaborate circuit to compensate for drift, or the use of a processing bandwidth wide enough to encompass the drift, which results in degraded sensitivity. It would be desirable for a radar altimeter to provide a simpler means for compensating for drift that does not result in degraded sensitivity.
A radar altimeter for determining altitude of an air vehicle with respect to ground comprises a digital sequencer for digitally modulating a first signal. A transmitter coupled to the digital sequencer transmits a radar signal including the modulated first signal toward the ground. A receiver receives a reflected radar signal from the ground. The received radar signal includes the modulated first signal. A digitizer coupled to the receiver generates digital samples of the modulated first signal. A digital signal processor coupled to the digitizer receives digital samples of the modulated first signal from the digitizer, demodulates the received digital samples, processes the demodulated digital samples and outputs altitude data based on the demodulated digital samples.
The radar altimeter of the present invention digitizes control and signal processing functions, allowing the system to be reduced to a single digital signal processor (DSP) chip, a sequencer chip, and a couple of radio frequency (RF) hybrid circuits. The radar altimeter compensates for frequency drift of the transmission signal with respect to the center frequency of processing filters by generating an intermediate frequency transmission signal from the digitizer sampling frequency source. Therefore, the IF transmission signal drifts with the sampling frequency and processing filter center frequency.